Enzyme/carbon structure composites can be used in a wide variety of applications, such as biosensors, biofuel cells, enzyme columns, ELISA kits, bioremediation devices, antifouling agents, and ibuprofen production, according to their purpose of use. In these applications, the carbon structures have various shapes, for example, carbon nanotubes, carbon nanorods and carbon nanowires.
For the use of a conventional enzyme/carbon structure composite, for example, in a fuel cell electrode or a biosensor, an enzyme should be immobilized on the surface of carbon structures. Due to the absence of functional groups capable of chemically bonding to the enzyme on the surface of the carbon structures, covalent bonds are formed between the enzyme and the carbon structures by modifying the surface of the carbon structures, followed by post-processing to produce the enzyme/carbon structure composite. Specifically, the right hand side of FIG. 1 shows a method for producing the conventional enzyme/carbon structure (e.g., carbon nanotube) composite. As shown in FIG. 1, the surface of the carbon structures is oxidized by treatment with a strong acid to form reactive groups capable of covalently bonding to the enzyme thereon. Thereafter, the surface-modified carbon structures are treated with a linker, such as EDC/NHS, and the enzyme is added to form covalent bonds between the carbon structures and the enzymes.
A larger amount of the enzyme can be immobilized on the surface of the carbon structures when covalent bonds are formed therebetween than when the enzyme is simply adsorbed onto the carbon structures. In addition, the formation of covalent bonds ensures relatively high stability and efficiency. However, the surface modification of the carbon structures for the formation of covalent bonds leads to a marked reduction in the electrical conductivity of the carbon structures. Further, the enzyme/carbon structure composite fails to achieve noticeable stability for successful application to enzyme fuel cells and biosensors, such as glucose sensors, that require repeated long-term use in various environments. The reason for this failure is known to be because the enzyme immobilized by only one or two covalent bonds is prone to denaturation over a long time or under rigorous conditions.